In general, continuous ink jet printing apparatus have an ink cavity to which ink is supplied under pressure so as to issue in a stream(s) from an orifice plate in liquid communication with the cavity. Periodic perturbations are imposed on the liquid stream(s), e.g. vibrations by an electromechanical transducer, to cause the stream(s) to break up into uniformly sized and shaped droplets. A charge plate is located proximate the stream(s) break-off point to induce an electrical charge to selected droplets in accord with a print information signal and charged droplets are deflected from their nominal trajectory. In one common (binary) printing mode, charged droplets are deflected into a catcher assembly and non-charged droplets proceed to the print medium.
The components described above (particularly the orifice plate and charge plate) should be precisely sized and positioned to achieve accurate placement of droplets on the print medium or on the catcher face. However, even after such careful manufacture, significant problems often are presented at each operational start-up of ink jet printers. First, any dried ink residue remaining from previous usage presents serious problems. For example, if such residue is on the charge plate it can cause shorting of the charge plate elements to ground or to each other, or cause improper charging of droplets, or interfere with the droplet trajectory. If the residue is on the lower print head structure (e.g. the operative catcher surface), it can cause ink splatter. Also, it is quite difficult to initiate the continuous droplet stream(s) along their nominal trajectories without some initial jet instability causing a partial wetting of the charge plate.
Prior art solutions to avoid charge plate shorting due to ink contamination have included: (i) manually cleaning the charge plate; (ii) providing a nearly instantaneous negative pressure at shut-down to avoid creating residue on he lower print head; (iii) moving the lower print head charge plate structure away from its operative position at start-up and (iv) providing a rapid pressure pulse in the image bar to force an initially straight start for the ink jets.
These solutions are all useful, but not without related difficulties or disadvantages. Manual cleaning of the charge plate is not desirable, particularly for office environment applications. Moving of the charge plate to avoid wetting during start-up adds mechanical complexity and causes great potential for inaccuracy in its proper alignment with the upper print head assembly's orifice plate. Using the "water-hammer" approach to achieve instantaneous start-up of the jets in their printing trajectory requires an extremely fast-actuation solenoid valve and rigid conduits. Also, this approach is unreliable in constructions where jet-to-electrode clearances are very small. Instant shut-down of the jets to avoid ink contamination on the charge plate has similar disadvantages and, in itself, will not solve the problem of accumulated residue on the lower print head structure.
U.S. Pat. Nos. 4,591,870; 4,600,928; 4,623,897 and 4,626,869 describe a series of procedures for improving the storage, start-up and periodic maintenance of continuous ink jet printer systems. These include wet storage of the print head, an ink flushing of the critical print head surfaces (e.g. orifice plate, charge plate and catcher surfaces), forced air skiving of liquid from those surfaces and subsequent condensate washing and drying of the surfaces. The start-up and maintenance of such printers is improved greatly by using all or even some of these procedures.
One potential problem has been found to exist in regard to the air skiving procedures described in U.S. Pat. No. 4,623,897. Specifically, air skiving has been effected by sealing the drop egress passage of the print head to a printer home station and forcing relatively high pressure air into an upper print head chamber. The pressurized air in the chamber flows into the drop egress passage, skiving ink off the critical surfaces, and discharges through the home station drain. This technique has been found to involve difficulties and costs in forming and maintaining effective seals between the print head and home station. Also, all print head frame portions that cooperate to enclose the print head chamber must be highly airtight. If air leaks are present in the frame, ink will be sprayed into the printer mainframe by the high air pressure provided for such skiving. This would be very undesirable.